A399-401 144MHz images

To examine the diffuse emission and the AGNs in Abell 399 and Abell 401, we used deep (~40h) LOw-Frequency ARray (LOFAR) observations with an improved direction-dependent calibration to produce radio images at 144MHz with a sensitivity of {sigma}=79uJy/beam at a 5.9{deg}x10.5{deg}resolution. Using a point-to-point analysis, we searched for a correlation between the radio and X-ray brightness from which we would be able to constrain the particle reacceleration mechanism.

Our radio images show the radio bridge between the radio halos at high significance. We find a trend between the radio and X-ray emission in the bridge. We also measured the correlation between the radio and X-ray emission in the radio halos and find a strong correlation for Abell 401 and a weaker correlation for Abell 399.

On the other hand, we measure a strong correlation for the radio halo extension from A399 in the northwest direction. With our deep images, we also find evidence for AGN particle injection and reenergized fossil plasma in the radio bridge and halos. We argue that second-order Fermi reacceleration is currently the most favored process to explain the radio bridge.

In addition, we find indications for a scenario in which past AGN particle injection might introduce significant scatter in the relation between the radio and X-ray emission in the bridge, but may also supply the fossil plasma needed for in situ reacceleration. The results for Abell 401 are also clearly consistent with a second-order Fermi reacceleration model. The relation between the thermal and nonthermal components in the radio halo in Abell 399 is affected by a recent merger.

However, a strong correlation toward its northwest extension and the steep spectrum in the radio halo support an origin of the radio emission in a second-order Fermi reacceleration model as well. The evidence that we find for reenergized fossil plasma near Abell 399 and in the radio bridge supports the reacceleration of the fossil plasma scenario.